The present invention relates to a technology for improving exhaust gas purification performance at a cold condition for a diesel engine.
In a diesel engine an air excess rate is controlled by a throttle valve, an EGR valve and so on so that the air excess rate equals a target value corresponding to an engine operating condition. However, since a temperature in an engine combustion chamber is low in a cold condition immediately after the engine has started, a combustion temperature in the engine combustion chamber is also low and more hydrocarbons (HC) emit from the combustion chamber.
EGR gases (exhaust gases) contain many components having a high specific heat such as carbon monoxide (CO), which reduce the combustion temperature. Therefore, the EGR is undesirable. Namely it is undesirable to control the air excess rate to equal the target value set corresponding to the engine operating condition when the engine is operating in the cold condition as contrasted with operation in a warm condition.
Japanese Unexamined Patent Publication No. 8-74676 discloses that HC emitted from the engine combustion chamber is restricted by decreasing an opening angle of the EGR valve in the cold condition for a larger air excess rate. The entire contents of this publication are incorporated herein by reference.
However, in the above noted technology the air excess rate is increased in the cold condition only for purification in the exhaust gases from the engine combustion chamber and it raises the following problems.
Vehicle engines are provided with an exhaust gas purification mechanism in an exhaust system therein such as a catalyst and a filter which purify exhaust gases from the engine combustion chamber. The exhaust gas purification mechanism can purify exhaust gas components at an activated state thereof wherein an exhaust gas temperature is high and can not purify them at a non-activated state thereof wherein it is low.
Accordingly, when the engine is cold, a temperature of the exhaust gas purification mechanism is low and it is in a non-activated state, and the exhaust gas components are emitted into an atmosphere without purification until the temperature of the exhaust gas purification mechanism gets high. An air excess rate has an inverse relationship with the exhaust gas temperature, such that the exhaust gas temperature is higher as the air excess rate is smaller, and the rise of the air excess rate brings about lowering the exhaust gas temperature.
Accordingly, when the air excess rate is controlled to be high in the cold condition as in the above-mentioned, HC emitted from the engine combustion chamber is restricted, but on the other hand, raising the temperature of the exhaust gas purification mechanism takes longer due to rising the air excess rate, thereby demanding more time for activate the exhaust gas purification mechanism.
There are some cases, therefore, where during warning the engine and the exhaust gas purification mechanism a total quantity of the exhaust gas components emitted from exhaust gas purification mechanism (to an atmosphere) increases rather than decreases.
One aspect of the present invention, in view of the foregoing problem, properly controls an air excess rate in a cold condition to decrease a total quantity of HC emitted into an atmosphere as much as possible during warming of an engine and an exhaust gas purification mechanism.
Therefore, the present invention sets a target air excess rate based upon an engine operating condition detected by a sensor, wherein in a cold condition (before the engine is not yet warmed), when an engine power is more than a predetermined value, the air excess rate is controlled to be equal to a first air excess rate greater than the target air excess rate. When the engine power is less than the predetermined value, the air excess rate is controlled to be equal to a second air excess rate smaller than the first air excess rate.
The other objects and features of the present invention will be understood from the following description with accompanying drawings.